Using a keyword too many times to try to cheat search engines into ranking your hub higher, therefore earning you more money.
You can inject your keyword you would like to rank for a few times within your article; this is common SEO practice. The term keyword stuffing, however, is stuffing the keyword into an article dozens of times, along with variations of that keyword. It is no longer effective in nearly all search engines, as their algorithms are engineered now to detect keyword stuffing.
If you'd like to rank better for whichever article you are writing, simply include the main keyword of the post two or three times inside of your article and you will see good results. Also, articles over 500 words tend to do very well in Google search results.